Collaborative Research: Linking Deep and Shallow Crustal Processes in a Continental Arc, North Cascades, Washington

大陆间碰撞形成的山带和挤压形成的大陆火山弧具有大规模的相似之处，包括地壳明显变厚。在过去的十年里，许多研究人员试图用部分熔融的中下地壳流动模型来解释碰撞山带的大规模模式，比如喜马拉雅山脉。相比之下，很少有关于大陆弧的地壳流动的研究。华盛顿州的北喀斯喀特山脉是一个白垩纪晚期（大约9000万年前）被挖掘出来的弧形地带，其中包括曾经在35-40公里深处的岩石，现在已经暴露在地表。该山脉还显示了始新世（约5000万至4500万年前）邻近的非海相沉积盆地形成过程中隆起和侵蚀的证据；这些盆地充满了被侵蚀的碎屑。一个跨学科的地质学家小组正在进行实地结构研究和实验室分析工作（地质年代学、岩石学、微观结构分析），以确定喀斯喀特的变质作用、部分熔化、深地壳岩石的挖掘和盆地形成和演化的时间。这些研究的目的是提供类似于玻利维亚和阿根廷现代安第斯山脉的Altiplano地区的大陆弧的构造和崩塌的观点。大部分研究都集中在北喀斯喀特的斯卡吉特片麻岩复合体上。这些岩石记录了部分融化的历史，这可能发生在山区的挖掘过程中。片麻岩部分被类似滑脱断层的低角度构造所包围。这些杂岩的流动与边界单元之间的关系以前没有研究过，但可以提供深、浅地壳过程耦合或解耦的信息。在一定的构造深度范围内，正在分析变形的运动学，以确定地壳流动的方向和条件，以及流动是否发生在与北美板块边界边缘平行或倾斜的通道中。北喀斯喀特两侧的始新世盆地包含由走滑和正断层体系的河流沉积的厚沉积层序。火山凝灰岩与沉积岩的夹层有助于确定盆地形成的时间和沉积速率。大量始新世岩脉侵入结晶岩和盆地。对岩脉进行测绘和定年的一个主要目的是在发掘山带和盆地形成过程中提供区域变形的记录。这些构造的取向可以与混杂岩记录的流动方向进行比较。本研究涉及三名研究生、本科生研究助理和暑期实习生。主要研究人员正在进行北喀斯喀特的实地考察，来自三所大学的学生都参与其中，并在互联网上发布了一份与这次考察和研究结果相关的插图、数据丰富的实地考察指南。

Mountain belts formed by collisions between continents, and continental volcanic arcs constructed during compression, have large-scale similarities, including markedly thickened crust. In the past decade, many researchers have tried to explain the large-scale patterns of collisional mountain belts, such as the Himalayas, using models of flow of partially molten mid- to lower crust. In contrast, there have been few studies of crustal flow in exhumed continental arcs. The North Cascades, Washington, is an exhumed Late Cretaceous (circa 90 million years ago) arc, which includes rocks that were once at depths of 35-40 km and are now exposed at the surface. This mountain range also displays evidence for uplift and erosion during formation of adjacent non-marine sedimentary basins in the Eocene (approximately 50-45 million years ago); these basins are filled with the eroded debris. Field-based structural study and laboratory analytical work (geochronology, petrology, microstructural analysis) are being carried out by an interdisciplinary team of geologists to determine the timing of metamorphism, partial melting, exhumation of deep crustal rocks, and basin formation and evolution in the Cascades. These studies are aimed at providing a view of the construction and collapse of a continental arc analogous to the Altiplano region of the modern Andes in Bolivia and Argentina. Much of the research is focused on the Skagit Gneiss Complex of the North Cascades. These rocks record a history of partial melting, which probably occurred during exhumation of the mountain belt. The gneiss is bounded in part by low-angle structures that resemble detachment faults. Relationships between flow of these migmatites and bounding units have not been previously studied, but can provide information about the coupling or decoupling of deep and shallow crustal processes. Kinematics of deformation are being analyzed over a range of structural depths to determine the directions and conditions of crustal flow, and whether flow occurred in a channel parallel or oblique to the edge of the North American plate boundary. Eocene basins that flank the North Cascades contain thick sedimentary sequences deposited by rivers in a system of strike-slip and normal faults. Volcanic tuffs interlayered with the sedimentary rocks facilitate dating of the time of basin initiation, and the rates of deposition. Vast swarms of Eocene dikes intrude both crystalline rocks and basins. A major goal of the mapping and dating of the dikes is to provide a record of regional deformation during exhumation of the mountain belt and basin formation. The orientation of these structures can be compared to the flow directions recorded by the migmatites. This research involves three graduate, undergraduate research assistants, and summer interns. The principal investigators are conducting a field trip to the North Cascades involving students from all three universities, and are publishing an illustrated, data-filled field guide related to the trip and the research results on the internet.